1. Field of the Invention
The invention is related to optical signal control, and in particular, to using birefringence to control the output of an optical coupler.
2. Description of the Prior Art
High speed optical switches and wavelength routers are two very important components in broadband optical networks. Polarization independence is very important for these components, because the polarization of optical signals at remote nodes of an optical network is usually random.
Technologies currently enjoying widespread use for multi-gigabit per second modulation include the Mach-Zehnder modulator (MZM) and the electroabsorption modulator. The lithium-niobate based MZM is a low-loss broadband device based on the Pockels effect. This device is not polarization independent. It requires linearly polarized optical signals with the polarization direction aligned to the principal axis of the MZM. This dependence on linear polarization poses a serious constraint in network functions because of the random polarization of these optical signals. A polarization-independent MZM has been demonstrated, but the complex structure and high drive voltage make them hard to implement. The second limitation of the MZM is that the wavelength dependence of its transfer function when used as a tunable filter is always sinusoidal.
At present there is a need for an optical device that is polarization independent, and that can be controlled at ultra high speeds with a reasonable voltage.